There is an increasing need for protection against forgery of documents using copiers, scanners, and various printing techniques. Watermarks have traditionally been used in paper and cloth materials to show origin or originality of the material. Watermarks traditionally were formed in a paper made of wood or rag pulp by forming an imprint or embossed image on the screen used to dry the paper. Watermarks today are formed by the same embossing or imprinting method, as well as by inks viewable only on copying, such as on checks and other security instruments. Watermarks are used to prevent copying or tampering with original materials.
For preventing copying or tampering, other materials have also been used. For example, variable security markings, including but not limited to diffraction gratings, holograms, interference coatings, metameric inks, and polarization coatings, have been widely adopted. These security markings vary in color or brightness, depending on the conditions under which they are illuminated and observed. Many of these security markings can be easily duplicated, so technologies that deliver high-level security are demanded.
U.S. Pat. No. 6,061,122 to Hoshino et al. discloses an optical identification system using high-polymer cholesteric liquid crystals, wherein the authenticity of the system is determined by recognizing optical properties of the cholesteric liquid crystal material that are substantially unaffected by an electric or magnetic field. The ability of the liquid crystal to reflect light at a certain wavelength, wherein the wavelength is dependent on the incident angle of the light, enables the formation of a holographic-like image that is difficult to reproduce.
U.S. Patent Application Publication US 2003/0173539A1 to Muller-Rees et al. discloses a security marking comprising liquid crystalline material with chiral phase, wherein the security marking is invisible to the eye, and the properties of the liquid crystalline material with chiral phase can be detected with the aid of detection devices, such as a polarizer or color filter.
U.S. Pat. No. 6,580,481 to Ueda et al. discloses an information recording/displaying card including an invisible written information area and a visible written information area. The invisible written information is recorded to be invisible, and can be accessed by a second device for reading. The invisible information is recorded on a magnetic recording layer, a write-once optical recording layer, or in an integrated circuit memory in which information can be recorded and erased.
The above-described markings either require special equipment to retrieve hidden information, or are difficult to produce. There is a need for a watermark that is easy to form and difficult to reproduce.